1. Field of the Invention
The present invention pertains to biometric imaging technology, and in particular, to live scanning of fingerprints and/or palm prints.
2. Background Art
Biometric imaging systems may include, but are not limited to, fingerprint imaging systems and palm print imaging systems. Such print imaging systems are also referred to as scanners or live scanners. Conventional live scanners use light to detect an image of a fingerprint and/or palm print. One or more fingers or palms are placed on a platen. An illumination source illuminates the underside of the platen. An image representative of valleys, ridges, or other characteristics of a fingerprint or a palm print is then detected by an image sensor, such as, a solid-state camera.
The illumination source must have sufficient power so that a high quality image representative of a print is detected by the image sensor. Often the optical system employed in a print scanner is telecentric which further increases the power required at the illumination source. In a telecentric system an aperture is used to limit light passing through the optical system. In this way, only light rays traveling within a range of angles at or near a direction along an optical axis are detected. Such telecentricity improves the image quality and reduces blurring, but has a drawback in that increased power is required at the illumination source to ensure that sufficient light passes through the aperture of the optical system to the image sensor.
Collimated light has been used in an illumination source for a print scanner to reduce the power required. A collimated lens collimates incoming light rays so that collimated light rays exit which travel parallel to one another. Because the rays are made parallel they travel efficiently through a telecentric optical system. FIG. 1A shows an example of a collimated light source lighting technique. A single discrete light source 110 emits light over an area as indicated by example rays 112. The actual emission area depends upon the type of emitter and other factors such as whether a lens, light guide or other optical element is provided to focus or guide the emitted light. A subset 115 of rays 112 are collimated by collimating lens 120 and emerge as parallel rays 125. The subset rays 115 are those rays within an angular range A at the focal point of collimating lens 120 as shown in FIG. 1A.
Collimated illumination source lighting techniques provide a relatively efficient, low power source especially in a telecentric optical system, but lack sufficient good grey scale shading for uses in high-quality fingerprint and/or palm print scanners. FIG. 1B shows an example conventional fingerprint or palm print scanner using collimated illumination in a telecentric optical system. A single discrete light source 110 emits red light which is collimated by collimating lens 120 before illuminating a prism 130. In this case, a top surface of prism 130 serves as a platen. The collimated light illuminates the underside of prism 130. When one or more fingers or palms are placed on prism 130, an image representative of valleys, ridges, and other characteristics of a fingerprint or a palm print is then focused by imaging lens 140 to aperture 150, and then further focused by imaging lens 160 to a focal plane/sensor plane 170. One or more image sensors, such as, solid-state cameras (CCD or CMOS cameras) detect the image. The use of collimated light improves the efficiency in which light travels through the telecentric optical system from the collimating lens 120 to the prism 130 and eventually through aperture 150 to an image sensor at focal plane/sensor plane 170.
While the collimated light is efficient, such illumination can produce print images having limited or no grey scale shading because the light rays incident on a platen are generally parallel. Such parallel rays are then either reflected or absorbed by a biometric object, such as, a finger or palm ridge. This tends to create a high contrast print having essentially black and white values only and no grey shade values. Such high contrast prints (also called binary prints) do not provide the full spectrum of grey shades required for many applications such as, forensics, law enforcement, security, and anti-terrorism. This can result in unacceptable images being captured especially in cases where significant pressure is placed on a finger or palm during the live scan.
As an alternative to collimated lighting techniques, print scanners have used diffuse source lighting. Diffuse light includes rays traveling in many different directions which tends to provide a flat, uniform illumination with good fill. In a print scanner, such diffuse light allows good grey scale shades to be detected since light traveling in a number of different directions is incident on a platen. One disadvantage is the illumination power required is high, especially in a print scanner having a telecentric optical system. The high power required means more light sources need to be used which, among other things, increases cost and the amount of heat generated.
FIG. 2 shows an example print scanner using a diffuse light source 205 that illuminates prism 130. Diffuse light source 205 includes a discrete emitter array 210 and a diffuser 220. Discrete emitter array 210 is made up a number of evenly spaced light emitting diodes that emit red light. As shown schematically in FIG. 2, diffuse light source 205 is an inefficient light source for generating an image and passing the image to an image sensor in a telecentric system. Diffuse light travels randomly or in different directions and is not transmitted through an entire telecentric system. For instance, much of the light is blocked by aperture 150. Such inefficiency in illumination of a print scanner is undesirable as it increases the number of emitters needed in array 210 and the power requirements of array 210. This problem is even more acute for telecentric print scanners where flat, uniform illumination is needed across a relatively large platen, such as, a platen big enough to allow capture of images for a roll print or slap print of one or more fingers, or a palm print.
In sum, the above drawbacks of using either collimation or diffuse illumination approaches in a print scanner need to be overcome. Other needs also exist in print scanner illumination. These needs include: a need for improved diffuse illumination of a print scanner, a need for providing a greater dynamic range of grey shading output from a camera, and a need for providing more uniform illumination from an illumination source array with flexible control.